In small gas turbine engines, the shrouds located around the turbine blades are generally not provided with a layer of abradable material, as is the case for some larger engines. Reasons for this include the facts that large engines may have more carcass distortions and more misalignment between centerlines of the rotor and shrouds. A distortion or misalignment may cause a localized rub between a shroud segment and all blades. Without an abradable system, this may leave a relatively large gap around the periphery of the rotor and reduce the efficiency of the engine. Smaller engines take advantage of having less carcass distortions and misalignments by designing to have tighter tip clearances. One method of achieving tight tip clearances on smaller engines is to machine blades to their final dimensions so that the designed tip clearance is achieved even without a running-in period.
Whenever parts are machined to their final dimension, for instance using a grinder, some material is removed. Since the parts are coated with one or more protective layers before the final machining process, removing more material than the thickness of the protective layer or layers will leave the base material exposed. The exposed areas will then be prone to oxidation. Oxidation is particularly severe at the edge of the pressure side of blades This ultimately results in a premature wear of the blades.
Accordingly, there is a need to provide an improved way of protecting from oxidation the surfaces of the blades that are machined because their base material is exposed once machined to their final dimension.